1. Field of the Invention
This invention relates to an aqueous coating composition for use in the manufacture of precoated sheet metal and more particularly, to an aqueous coating composition for the manufacture of precoated metal sheets which forms a coating having improved adhesion, chemical resistance and water resistance so that the precoated metal sheets are useful as exterior members of electric appliances and buildings.
2. Prior Art
The precoated metal technology is to previously coat metal sheets such as iron sheets, aluminum sheets, galvanized sheets, and stainless steel sheets with paint before the metal sheets are mechanically worked into complex shapes as by die pressing, bending, embossing, rolling, and drawing. This technology has been widely employed for decades because surfaces of complex shape can have a uniform coating of paint.
In the precoated metal technology, coating is followed by working and assembling steps. Coating compositions used in the precoated metal technology are required to tolerate impacts encountered upon shaping as by bending, rolling and embossing and possess firm adhesion to and coverage on sheet metal. They are sometimes required to be resistant to mar and stain. For outdoor use, they are required to be weather resistant.
Prior art metal precoating compositions are based on aminoalkyd resins, aminoacrylic resins, and epoxy resins. Recently used are polyurethane resins from which precoated metal featuring improved working is obtained. Most metal precoating compositions are of the solvent type using organic solvents such as toluene, xylene, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate, cyclohexanone, and cyclohexanol. There is an increasing demand for aqueous coating compositions.
Aqueous coating compositions which have been commonly used contain acrylic copolymer resins, polyester resins and polyurethane resins each having a carboxyl group as a binder. These aqueous coating compositions, however, are less suitable for the manufacture of precoated metal. After the composition is applied and dried to a metal sheet, the coated metal sheet is mechanically shaped whereupon the coating can be peeled from the underlying metal sheet. Upon exposure to solvent or hot water, the coating can be dissolved. These undesirable phenomena are caused by the hydrophilic nature of a carboxyl group in the binder resin in the aqueous coating composition. The problems can be overcome to some extent by selecting an aqueous resin having a low acid value to reduce hydrophilic nature. The aqueous resin having a low acid value, however, is less soluble or dispersible in water and in an extreme event, does not satisfy the requirements as paint of pigment dispersion, an appropriate viscosity and transition. It is then quite difficult to provide a practically acceptable coating composition. There is a need to have an aqueous metal precoating composition free from the above-mentioned problems.